Deficiency Symptoms

 




General Role or Functions of Essential  Elements
1) Constituents  of Organic  Molecules: Depending  upon the organic compound formed, mineral elements  are of two types:_
(a) Framework Elements: They produce cell walls and storage  products of plants, i.e., C, H, O. 
(b) Protoplasmic  Elements: The elements  produce protoplasmic  constituents  like proteins,nucleic acids, chlorophyll,  cytochromes, ferredoxin, i.e., C, H, O, N, S, P, Mg, Fe. 

2. Osmoti5 Potential  : Most of the osmotic potential  of cell sap is due to inorganic salts. 

3. Movements: Free K+ takes part in stomatal and other  turgo movements   

4. Buffer Action : Weak acids and their salts function as a buffer against  changes of pH. 

5. Oxidation Reduction  System  : Metals with variable  valency act as electron carriers, e.g.,  Iron and Copper. 

6. Balancing   Elements  : Ca²+ ,Mg²+ and K+ minimise the toxic effect of heavy elements.  

7. Permeability  :Sodium, Potassium  and some other  monovalents increase  membrane permeability  while Calcium  and other diavalents  decrease it. 

8. Calalytic  Effects: Many enzymes require mineral elements as K+ is known to be cofactor of some 40 enzymes. 

9. Phloem Transport  :  Boron and Potassium  are involved  in the translocation.  

Deficiency  Symptoms: 

Deficiency  symptoms are extremely  visible deformities or abnormalities  which are produced due to absence  or deficiency  of some essential  nutritive substance. They are also called hunger signs. 


Deficiency  symptoms  are studied by means of culture experiments.Rapidly growing plants which develop characteristic symptoms  are used in culture experiments. They are called test (= indicator) plants. They are then grown in soil under test in small pots. The results are compared to know the deficiency  elements.  

The concentration  of the essential  element below which plant growth  is retarded is termed as critical  concentration.  The element is said to be deficiency  when present  below the critical  concentration.  

The parts of the plants that show the deficiency  symptoms also depend on the mobility  of the element in the plant. For elements  that are actively mobilized  within the plants and exported to young developing  tissues,  the deficiency  symptoms tend to appear first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium  and magnesium are visible  first in the senescent leaves. In the older leaves, biomolecules containing   these elements are broken down, making these elements available  for mobilizing  to younger leaves. 

   The deficiency  symptoms  tend to appear first in the young tissues whenever the elements are relatively  immobile and are not transported  out of the mature organs, for example,  elements  like sulphur and calcium  are a part of the structural component of the cell and hence  are not making these  elements  available for mobilizing  to younger  leaves. 

  The deficiency  symptoms  tend to appear   first in the young  tissues  whenever  the elements  are relatively  immobile and are not  transport  out of the mature organs, for example,  elements  like sulphur and calcium  are a part of the structural  component of the cell and hence are not easily released. Iron, boron, manganese and copper  are also among  immobile elements. 

Common   Deficiency  Symptoms:

1) Chlorosis : Nondevelopment or loss of chlorophyll  .

2) Mottling: Patches of green  and  nongreen areas.

3) Necrosis : Death of tissues. 

4) Stunted Growth: 

5) Abscission: Premature fall of flowers and fruits.

6) Leaf fall 

7) Leaf Curl 

(8) Wilting : Loss of turgor, 

9) Internal Cork 

10) Internal or Heart Rot : Softening  or rotting  of internal  tissues. 

11) External Cracks 

12) Die_ Back : Killing  of shoot apex.

13) Little leaf Disease: Leaves  are quite small in size. 

14) White Bud: Chlorosis affecting young leaves as well as buds so that the latter are whitish instead of greenish colour.

Some Common   Deficiency  Symptoms: 

1) Chlorosis : N, K,  Mg,  S,  Fe,  Mn,  Zn,  Mo.

2) Necrosis : Death of tissues,Ca, Mg, Cu, K.

3) Inhibition  of Cell Division: N, K,  S,  Mo. 

4) Late Flowering  : N, S, Mo.

Absorption  Minerals

The most active areas  of the root  for mineral absorption  are the zones of elongation  and root hair. The rate of mineral absorption  is usually  independent  of its concentration in the soil. They  are absorbed  as ions. Ions  are accumulated  by the plants against  their concentration  in the soil.  Two main phases are involved  in the uptake of mineral ions. In the first phase or physical uptake, tissues kept in mineral solutions show an initial  uptake of ions into the free space or outer space of the cells. In the second phase or metabolic  uptake, ions are taken in slowly  into the inner  space of cells. The outer  space includes the intercellular  space and cell wall, and the inner space refers to the cytoplasm  and the Vacuole. Entry of ions the outer space seems to be  passive whereas  entry into and exit from the inner, space, usually  requires energy  and is, therefore, an active  process.The movement  of ions  is usually termed as flux. The movement  into the cell is influx and the  outward movement  is efflux. 

Passive Mineral Absorption

The plant does not spend energy.  It takes place by the following  methods:_ 


1.  Diffusion: It is of two types : 

(i) Passive diffusion : Membrane  has either small pores or its matrix is soluble to diffusing particles.  No energy or any permeases  are required. It operates  through physical  forces like chemical potential,  electrochemical  gradient, hydrostatic  pressure,  diffusion pressure gradient. Water , O2, CO2, Na  ( in case of  Nitella) are known to follow  passive  diffusion.  

(ii) Facilitated  Diffusion: Membrane  possess  permeases or special  protein particles that facilitate the passage of specific  substances  through them. No energy is  involved. 


2. Mass Flow : The minerals are swept into the root during  the rapid absorption  of water as the one taking place during periods of high  transpiration. It occurs due to passive absorption of ions  by free diffusion  into the apparent  free space of a tissue.  This process occurs due to transpirational pull in the absence of metabolic energy

3.  Donnan Effect and Equilibrium  : The Donnan effect and equilibrium  theory takes into account  the effect  of fixed or non_diffusible ions. Let us take, for example, a membrane  that is permeable  to some ions and not to others and that separates  the cell from the external medium. Let us suppose that on the inner side of this membrane  there is a concentration of  anions to which the membrane is impermeable  ( negatively charged proteins are examples  of fixed anions). Now, if the above membrane  is freely  permeable  to the cations and anions in the external solution, equal numbers of cations and anions from the external  solution  will diffuse across  the  membrane until an equilibrium  is established . Normally, this equilibrium  would also be electrically  balanced. Additional  cations are needed  to balance the negative  charges of the fixed anions  on  the  inner side of the membrane. The cation  concentration  would become greater  in the internal  solution  that it is in  the  external solution.  Because of the excess of negative  charges due to fixed anions, the concentration  of anions in the internal solution will be less than the concentration  of those ions in the external  solution. 

 The product of the anions and cations in the internal  solution  is equal  to that of the anions and cations in the external  solution,  the Donnan equilibrium  is attained according  to the following  equation: 







 The accumulation  of ions against a concentration  gradient  can occur without  the participation  of metabolic  energy  until a Donnan equilibrium  is reached. We should recognize,  that although  this mechanism  may not necessarily  occur in plant tissue as described  it serves as one  explanation to account  for the passive accumulation  of  ions against  a concentration  gradient  in response  to an electrochemical  potential  gradient.  

4. Ionic Exchange  : It is the exchange  of ions between  a cell of root and the external  medium. Cations ( e.g., K+, Ca²+, Mg²+) are  exchanged with H + ions while anions ( e.g., CI- NO3-, SO4²- ) are  exchanged  with OH  or HCO3 - ions. Ionic exchange between  root and soil  can occur by two methods: 

(a)  Carbonic Acid  Exchange  : CO2 produced  in resp dissolves in water to form carbonic acid. Carbonic acid dissociates  into hydrogen and bicarbonates  ions. 

H2O➕ CO2 ↔️ H2CO3↔️ H+➕ HCO3-

 The two types of ions get exchanged  with cations and anions found in the external  solution.  

(b) Contact Exchange  : H+, OH- or HCO3- ions absorbed  over the surface of root can directly  get exchanged  with similarly  ions presents over the  surface of soil particles  if their oscillation  volumes overlap.  



Role of Essential  Elements  and Deficiency  symptoms 


















Tiller: a side shoot arising at ground level. Generally  seen in grasses and cereals. 

Active Mineral Absorption  

It occurs against concentration  gradient.  Metabolic  energy   is used. The extra respiration  required  for providing  energy during absorption  of minerals  is called salt respiration.  Mineral absorption  against  concentration  gradient  immediately  comes to a stop if nitrogen is bubbled in the rooting medium. Rapid accumulation  occurred if oxygen is bubbled.  

 Active absorption  occurs through the agency of specific carriers  present in the plasma membranes of the absorbing  cells. These organic molecules  pick up ions from the outer surface  of plasma membrane and release the same on the inner surface. Another type of carriers are vesicles  or pinosomes. The letter are invaginations of the plasma membrane. The vesicles get filled minerals,  pinch off and enter the cytoplasm  where their membranes disintegrate  to release the contents. 

 Unlike ion channels, the carriers proteins  do not have pores. Ion channels  are transmembrane proteins that function as selective  pores for ions in passive diffusion. 


Ions traffic into the root:  Minerals absorbed by the root are carried to the xylem by two pathways_ apoplast  and symplast 


Transport of Minerals 

 The absorbed minerals pass radially inward into the tracheary elements  of the root. They reach leaves and other parts along with water. Xylem is the pathway for this transport. It was proved by  Stout and Hoagland.  They inserted paraffin  paper between  xylem and phloem  of the stem upto a distance of 23 cm. Radioactive  potassium or phosphorous  was  added to the rooting medium. Concentration  of radioactive  mineral was found out at various  levels. In the stripped  area radioactive  mineral was present only in xylem while in the unstripped area it  occurred  in both bark and wood. Xylem takes the minerals  mostly to leaves. From  leaves the mineral salts reach other parts of plant body through  phloem. It has been seen  that the rates at which inorganic  solutes are translocated  through  the xylem vessels,  correspond closely  with the rates of translocation of water. Thus solutes are carried along with the water, which is pulled up by transpirational pull. 

Nitrogen Nutrition 

Plants absorb nitrogen in the form of NO3- ( nitrate) or NH4+ ( ammonium  ion). They can absorb NO2- ( nitrite) also but it does not accumulate in the soil. Only a small quantity of nitrate or ammonium  is available  in the lithosphere. Nitrogen is the most critical  element. The reservoir pool is atmosphere. Molecular nitrogen cannot be utilized  directly  by plants. It has to be fixed or converted into compounds prior to utilization. There are two methods of nitrogen fixation _ abiological and biological. Abiological nitrogen  fixation is further of two kinds, natural and industrial.  

Natural Abiological  Nitrogen Fixation  : Atmospheric  N2 combines with O2 in the presence  of electric discharges,  ozonization  and combustion . Different types of nitrogen  oxides are produced. The nitrogen  oxides  dissolve  in water  and give rise  to hyponitrous, nitrous and nitric acids. They enter soil  alongwith rain water forming  hyponitrites, nitrites and nitrates. 





Industrial Abiological  Nitrogen  Fixation: Ammonia is produced  industrially  by direct combination of nitrogen  with hydrogen  ( got from water) at high  temperature and pressure . It is changes to various types of  fertilizers  including  urea. Industrial combustions, forest fires, automobile exhausts and power generating  stations are also sources of atmospheric nitrogen oxides. 

 Biological  Nitrogen Fixation  : It is performed by two types of prokaryotes , bacteria  and cyanobacteria. They include both free living  and symbiotic  forms. 

(a) Free Living Nitrogen  Fixing Bacteria: Azotobacter, Beijerinckia (both aerobic) and Clostridium are saprotrophic  bacteria that perform nitrogen fixation.  Rhodopseudomonas, Rhodospirillum ( anaerobic)  and Chromatium are nitrogen fixing photoautotrophic bacteria. Free living  nitrogen fixing bacteria  add 10_ 25 kg of nitrogen/ ha/ annum. 

(b) Free Living Nitrogen  Fixing  Cyanobacteria: e.g., Anabaena,Nostoc, Aulosira, Cylindrospermum, Trichodesmium. They add 20_ 30 kg of nitrogen  per hectare  of soil. They are also important  as they occur in water _ lagged  soils where denitrifying  bacteria can be active. Aulosira fertilissima is the most active  nitrogen  fixer in Rice fields while Cylindrospermum is active in Sugarcane and Maize fields. 

(c) Symbiotic  Nitrogen   Fixing Cyanobacteria  : Anabaena  and Nostoc species  are common  symbionts in lichens,  Azolla and Cycad roots. Azolla pinnata  ( a water fern)  has Anabaena  azollae  in its fronds. It is often  inoculated to Rice fields  for  nitrogen fixation.  

(d) Symbiotic  Nitrogen  Fixing  Bacteria: Rhizobium is nitrogen fixing   bacterial symbiont of papilionaceous roots like alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover  beans, etc. Sesbania rostrata has Rhizobium in root nodules and Aerorhizobium in stem nodules. Frankia is symbiont in root nodules of several  nonlegume  plants like Casuarina and Alnus ( Alder). Both Rhizobium and Frankia are free living  in soil, but as symbionts can fix atmospheric  nitrogen. Xanthomonas and Mycobacterium  form symbiotic  associated  with the leaves of Ardisia. 


Rhizobium  lives in the soil but is unable to fix nitrogen there. It enters the legume roots and develops a number of tubercles or nodules. Root nodules are small irregular  outgrowths of the roots. They are internally  pinkish due to the presence  of a pigment  called leghaemoglobin  or leguminous haemoglobin.  It is related to haemoglobin . The cells of the root  nodule bacteria are called bacteriods. Leghaemoglobin  is located between  bacteriods and the surrounding  host membrane. Leghaemoglobin is an oxygen  scavenger and protects the nitrogen fixing enzyme nitrogenase of the bacteriods. These microbes  live as aerobes under free_ living  conditions ( where nitrogenase  is not operational), but during nitrogen_ fixing events, they become  anaerobic. Nitrogen fixation  occurs under the control of plant nod  genes  and bacterial  nod, nif and fix gene clusters. 


  Formation of root nodules: When  a root hair of a leguminous  plant comes in contact with Rhizobium , it curls or is deformed. Specific  chemical  substances  secreted  by the bacteria are responsible  for the curling . At the site of curling  or deformation  of root hair, rhizobia invade the root  tissue and proliferate within the root hair. Some of the bacteria  enlarge to become membrane  bound  polyhedric structures called bacteriods. 


The infected host cells become tetraploid: The  bacteriods cannot multiply,  thus some bacteria remain untransformed,  which allows infection to spread. The plant responds by forming an infection thread, made up of the plasma membrane,  that  grows inward from the infected  cell of the host, separating  the infected  tissue from the rest of the plant. Cell division  is stimulated in the infected  tissue ( inner cortex and pericycle) and more bacteria invade the newly formed tissues. A combination  of cytokinine produced by the invading  bacteria and auxin produced by plant cells, promotes cell division and extension, leading  to nodule formation.  The nodule thus formed, establishes  a direct vascular  connection  with the host for the exchange  of nutrients.  The cells of infected zone are highly  enlarged  and vacuolated. They contain  bacteriods. 


Mechanism  of Nitrogen Fixation  : Nitrogen fixation requires: 

(i) A reducing  power like NADPH, FMNH2

(ii) A source of energy  like ATP

(iii) Enzyme nitrogenase and 

(iv) Compounds for trapping  ammonia  formed by the reduction  of dinitrogen. 

  Nitrogenase has iron and molybdenum. Both of them take part in attachment  of a molecule of  nitrogen (N2).  Bonds between  the two atoms of nitrogen become weakened  by their attachment  to the metallic  components.  The weakened  molecule of nitrogen is acted upon by hydrogen  from a reduced  coenzymes.It is toxic  in  even small quantities. The nitrogen fixes protect themselves  from it by providing  organic acids and forming amino  acids.  The reaction is as follows: 

N2 ➕ 8e- ➕8H+ ➕ 16ATP ➡️ 2NH3➕H2➕16ADP➕ 16Pi

 Thus  ammonia formation   requires very high input of energy ( 8ATP for each NH3 produced). The energy  is obtained  from respiration  of hist cells. 















Symbiotic  of  nitrogen  fixing organisms  hand over  a part of their fixed nitrogen to the host in return for shelter  and food. Free living nitrogen  fixes do not  immediately  enrich the soil. It is only after their death the fixed  nitrogen  enters the cycling pool. It occurs in two steps, ammonofication and nitrification. 
 
 Ammonofication is carried out by decay causing organisms  which act upon nitrogenous  excretions  and proteins of dead bodies of living  organisms, e.g., Bacillus, Actinomyces. Proteins are first broken up into amino  acids. The latter are dominated. Organic acids released  in the process are used by microorganisms  for their own metabolism. 

         






Ammonia does not remain in the gaseous state in the soil but is changed to ionic form ( NH4+). It can be used by plants directly provided  pH of soil is more than 6 and the plant contains  abundant  organic  acids. Unlike nitrates, very few plants can store  ammonium  ions (e.g., Begonia, Oxalis). Some of the ammonia volatilises and re_ enters  the atmosphere  but most of it is converted  into nitrate by soil bacteria. 

Nitrification  is the phenomenon  of conversion  of ammo nitrogen to nitrate nitrogen. It is performed in two steps_ nitrite formation and nitrate formation. Both the steps can be carried out by  Aspergillus flavus. In the first step, ammonium  ions are oxidised to nitrites by bacteria   Nitrosococcus and Nitrosomonas. In the second  step nitrites are changed  to nitrates, e.g., Nitrocystis, Nitrobacter. 







Most  of the bacteria performing nitrification ( e.g., Nitrosococcus, Nitrosomonas, Nitrobacter)  are chemoautotrophs. The nitrate thus formed is either made available  to the plant and is transported to the leaves  or is   converted to nitrogen  gas in the process of denitrification  by other micro_ organisms e.g., Pseudomonas and Thiobacillus. 

Nitrate Assimilation  

Nitrate is the most important source of nitrogen to the plants. However  it cannot  be used  as such   by  the plants.It  is first reduced to level of ammonia  before being incorporated  into organic compounds. Reduction of nitrate occurs in two steps: 

(i) Reduction of Nitrate to Nitrite: It is carried out by an inducible  enzyme nitrate reductase.The enzyme  is a molybodoflavoprotein. It requires a reduced coenzyme ( NADH or NADPH) for its activity. 


(ii)  Reduction of Nitrite: It is performed by enzyme nitrite reductase. The enzyme  is a metalloflavoprotein which  contains copper and iron. It occurs inside chloroplasts  in the leaf cells and leucocytes  of other cells. In contrast nitrate reductase is found attached loosely  to cell membrane. Nitrite reductase requires NADPH in illuminated  cells and NADP in others. The process of reduction also requires ferredoxin which occurs in higher plants mostly in green tissues. It is presumed  that in higher plants either nitrite is translocated  to leaf or some other electron donor ( like FAD  operates in unilluminated cells). 







Ammonia is not liberated.It combines with some organic acids to produce amino acids. 


Synthesis  of Amino Acids

It occurs mostly in the roots and leaves where nitrates are reduced. The cells  of these regions have to provide the necessary  organic acids which are mostly formed in Krebs cycle. As Krebs cycle occurs in mitochondria, the sites of amino acid synthesis  are also assumed to be the mitochondria. Amino acids  are formed  by the following  methods: 

(i) Reductive  amination: In the presence of dehydrogenase  and a reducing  power ( either NADH2 or NADPH2) , ammonia can combine directly  with a keyo acid to produce  an amino acid. The substrate  is generally  a_ ketoglutaric acid, an intermediate  of Krebs cycle. It gives rise to glutamic acid. 






Besides æ_ ketoglutaric  acid other  organic acids which undergo reductive animation  are oxaloacetate and phosphoenol pyruvate.  

(ii)  Transamination : It involves  the transfer of amino group from one amino  acid  to the keto  group of keto acid.  The reaction is carried out in the presence of an enzyme named transaminase. It requires coenzy pyridoxal phosphate, a derivative  of pyridoxine  or vitamin  B6. Teansamination reactions are reversible. Glutamic  acid is the main amino acid  from which other 17 amino acids are formed through transamination. 

(iii) Transformation: A number of amino acids  are produced  from others by chemical transformation  through  oxidation, reduction, condensation  etc. 

(a) Oxidation: e.g., formation of hydroxyproline from proline. 

(b) Reduction : e.g., aspartic acid loses oxygen  to form homoserine. 

(c) Condensation : e.g., two molecules  of glycine condense to form serine with the release  of a CO2 molecule . 

(iv) Catalytic amidation: Ammonia combine with catalytic  amounts  of glutamic acid in the presence  of ATP and enzyme glutamine  synthetase. It produces the amide glutamine. 

 Glutamine  reacts with æ_ Ketoglutaric  acid  in the presence of NADH or NADPH and enzyme glutamic acid synthetase to form two molecules  of glutamic acid.  









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